1. Field of the Invention
This invention relates to an electronic circuit board case, particularly to a case for an electronic circuit board that accommodates the electronic circuit board in an internal space defined by upper and lower case members and prevents cracking of soldered portions between the electronic circuit board and electronic components owing to thermal stress and/or vibration.
2. Description of the Related Art
An electronic circuit board housed in a case and mounted on a vehicle or the like is apt to experience cracking of soldered portions between the circuit board and electronic components mounted thereon, particularly between the circuit board and surface-mounted components without leads (leadless components), owing to vibration from outside and thermal stress caused by temperature change, more specifically warping of the circuit board caused by difference in coefficient of thermal expansion between the case and the circuit board. The circuit board is therefore fixed at multiple points by an upper case member or upper and lower case members so as to prevent circuit board warping.
FIG. 8 is a schematic view of an electronic circuit board case according to the prior art.
The case includes an upper case member 10 and a lower case member 12, both fabricated by press-forming thin metal sheet. An electronic circuit board 14 mounted with electronic components (not shown) is accommodated in the internal space defined by the upper and lower case members 10 and 12. More specifically, the lower case member 12 is fit into a lower open end of the upper case member 10 while accommodating the electronic circuit board 14.
The lower case member 12 is formed with a number of pedestals 16 and, at its edge portions, locally with lugs 18. The upper case member 10 is formed with screw holes 20 that align with the lugs 18.
After the electronic circuit board 14 has been placed on and fastened to the pedestals 16 by screws (not shown), the upper case member 10 is set in place from above, and the upper case member 10 and lower case member 12 are united by passing screws 24 through the screw holes 20 of the upper case member 10 and screwing them into lug screw holes 22 formed in the lugs 18.
As taught by Japanese Utility Model Registration No. 2586966, moreover, there is also known a technique of fastening the circuit board and upper and lower case members together with the edge portion of the circuit board clamped between the edge portions defining openings of the upper and lower case members.
Further, as taught by Japanese Patent Laid-Open Applications Hei 10(1998)-224057 (""057) and Hei 11(1999)-261254 (""254), for example, it is also known to fasten the circuit board to the case without screwing the circuit board and case together. Specifically, according to the technique taught by ""057, a number of projections are formed on the ceiling of the upper case member and on the floor of the lower case member, and the circuit board is immobilized or retained by clamping its surfaces between the two sets of projections. On the other hand, ""254 teaches a technique of fixing a circuit board by clamping its edge portion together with a cushioning material of rubber or the like between the edge portions defining openings of the upper and lower case members.
When the surface of a circuit board is retained at multiple points by use of screws, the circuit board can be firmly fastened to prevent warping and the occurrence of warping-induced cracking, but the mounting area of the circuit board is reduced and the fabrication steps required are increased in number and/or complexity. When a circuit board is fastened by clamping the entire length of its edge portion, thermal stress produced by warping and vibration cannot be absorbed and, depending on the use environment, stress produced by temperature change and/or vibration may be aggravated.
While the technique involving use of a cushioning material reduces vibration-induced stress, it does not overcome the problem of circuit board warping caused by temperature change. It is also disadvantageous in terms of number of fabrication steps and cost.
Further, as illustrated in FIG. 9, the action of thermal stress is relatively large around the circuit board fastening points (screw fastening regions), so that these regions are likely to experience heavy warping and, as a result, cracking. In the situation of an acceleration sensor (accelerometer) or other such device whose detection accuracy must be optimized, however, it is preferable to join the circuit board and the case solidly together so that vibration and shock are transmitted without damping and to install the device in the vicinity of a fastening point, such as at the edge portion of the circuit board. It is therefore preferable to join the circuit board and case firmly and to prevent cracking by ensuring that the circuit board does not warp in the vicinity of the fastening points. In the situation of mounting an acceleration sensor or the like, moreover, use of a cushioning material like rubber, which degrades detection accuracy, should best be avoided.
Further, in order to protect the case itself and the electronic circuit board housed therein from vibration and impacts received from the outside, the strength of the case should be increased to the greatest extent possible without increasing the weight of the case.
An object of the present invention is therefore to provide an electronic circuit board case that, without reducing the mounting area of the circuit board or increasing the number of fabrication steps, prevents warping of the electronic circuit board owing to thermal stress and/or vibration and thus prevents cracking of soldered portions, enables firm and reliable immobilization of the electronic circuit board, and achieves increased case strength.
For realizing this object, a first aspect of this invention provides a case for an electronic circuit board comprising an upper case member having a peripheral edge portion defining an opening, a lower case member having a peripheral edge portion defining an opening, the peripheral edge portions of the upper and lower case members being of complementary shape, fastening means for fastening the upper case member and the lower case member together with an edge portion of the electronic circuit board between the peripheral edge portions, and protrusions provided on inner side walls of the upper case member and lower case member, the protrusions being located to face one another in such a manner that they clamp a local edge region of the electronic circuit board clamped therebetween.
Since an edge portion of the electronic circuit board is clamped between peripheral edge portions of the upper and lower case members and a local edge region of the electronic circuit board is further clamped between the protrusions formed at opposing locations on the inner side walls of the upper and lower case members, the electronic circuit board can be firmly and reliably fastened to the case without reducing the mounting area of the circuit board or increasing the number of fabrication steps, and warping of the electronic circuit board can be prevented so as to preclude occurrence of cracking. In addition, the strength of the case is increased.
In a second aspect, the invention provides an electronic circuit board case, wherein flange portions are formed at corners of the peripheral edge portion of the upper case member, flange portions are formed at corners of the peripheral edge portion of the lower case member at locations opposite the flange portions of the upper case member, the fastening means clamps the electronic circuit board between the flange portions, and the protrusions formed at opposing locations on the inner side walls of the upper and lower case members clamp a local edge region of the electronic circuit board.
Since flange portions are formed to oppose one another at corners of the peripheral edge portions of the upper and lower case members, the fastening means clamps the electronic circuit board between the flange portions, and the protrusions formed at opposing locations on the inner side walls of the upper and lower case members clamp a local edge region of the electronic circuit board, the electronic circuit board can be still more firmly and reliably fastened to the case and the strength of the case can be further increased.
In a third aspect, the invention provides a case for an electronic circuit board, wherein the protrusions are formed integrally with flange portions as continuations thereof.
Since the protrusions are formed integrally with flange portions as continuations thereof, warping of the electronic circuit board in the vicinity of the fastening points between the electronic circuit board and case members, i.e., in the vicinity of screw fastening regions, can be effectively prevented. As a result, cracking in the vicinity of the screw fastening regions can also be effectively prevented.